Poor conditions such as an increasing BLER (Block Error Ratio, block error rate), air interface packet loss, and increasing retransmissions may occur on an LIE (Long Term Evolution, long term evolution) network in an adverse radio environment. As a result, a large amount of data is buffered by a UE (User Equipment, user equipment) in a transmitting window, a receiving window, and a reordering window of the RLC (Radio Link Control, radio link control)/PDCP (Packet Data Convergence Protocol, packet data convergence protocol) of the layer 2 protocol (L2, that is, the data link layer protocol). In addition, different implementations of user plane memory management and different implementations of interfaces between the L2 protocol and external modules may also result in increasing memory usage of a buffer (Buffer).
However, considering factors such as cost reduction, volume, and power consumption, the memory of an L2 buffer used by an LTE UE is limited at a certain level. In an adverse radio environment, the data amount of the L2 buffer may reach or exceed the upper limit of the memory of the buffer, that is, the memory of the L2 buffer overflows. In this case, when uplink and downlink data continues to enter the L2 buffer, memory allocation fails. Once memory allocation fails, the user plane data transmission is interrupted exceptionally, and a more serious system exception may occur.
In the prior art, for the interruption of user plane data transmission due to the memory overflow of the L2 buffer, the following two recovery mechanisms are provided.
As shown in FIG. 1, in a process of communication between a UE and an eNB (evolved NodeB, evolved NodeB), after the overflow of the L2 buffer occurs, the number of uplink retransmissions of an RLC PDU (PDU is short for Protocol Data Unit, protocol data unit) reaches the maximum, and an RRC (Radio Resource Control, radio resource control) reestablishment procedure is triggered; the reestablishment procedure fails, and the RRC releases all RBs (Radio Bearer, radio bearer), so that the memory of the L2 buffer is recovered. The specific process is as follows:
11. When the L2 buffer overflows, new upper layer uplink data cannot be received.
12. The PDU in the uplink transmitting window of the RLC is retransmitted after a t-PollRetransmit timer (timer of the RLC) expires.
13. Because the L2 buffer overflows, an L2 MAC (Medium Access Control, medium access control) cannot receive the PDU reported by a PHY (Physical Layer, physical layer), that is, the RLC cannot receive a status PDU sent from the NodeB.
14. After the t-PollRetransmit timer expires, the uplink PDU of the RLC continues to be retransmitted until the maximum number of retransmissions is reached.
15. The RLC reports radio link failure signaling RLC_ERR_IND to the RRC.
16. The RRC initiates an RRC connection reestablishment procedure as specified in section 5.3.11 of 3GPP TS 36.331, and starts the T301 timer.
17. The L2 executes the reestablishment procedure. The L2 data continues to be buffered on the PDCP according to 3GPP TS 36.322 and 3GPP TS 36.323. However, because the L2 buffer cannot be released, the L2 data retransmission function is still unavailable, and the RRC connection reestablishment request fails to be sent.
18. After the RRC T301 expires, all the RBs are released as specified in section 5.3.7.7 and section 5.3.12 of 3GPP TS 36.331.
19. All the buffers of the L2 are released, that is, the memories of the buffers are recovered.
As shown in FIG. 2, in the process of communication between the UE and the eNB, after the L2 buffer overflows, the L2 data transmission is unavailable, and the eNB actively releases the UE; the state of the UE is inconsistent with that of the eNB, and the UE RRC is finally triggered to release all RBs, so that the L2 buffer is recovered. The specific process is as follows:
21. When the L2 buffer overflows, new uplink and downlink data cannot be received.
22. The eNB detects that the UE state is exceptional, and initiates an RRC connection release message to release the UE actively.
23. Because the L2 memory overflows, the RRC connection release message cannot be received, and the UE is still in a connected state.
24. Because the eNB has released the UE and the state of the UE is inconsistent with that of the eNB, the PHY detects an out-of-synchronization event and reports the event to the RRC; or the TA timeout may occur on the MAC, and the PHY reports the TA timeout to the RRC.
25. The RRC initiates a connection reestablishment procedure as specified in section 5.3.11 of 3GPP TS 36.331, and starts a T301 timer.
26. The L2 executes the reestablishment procedure. The data should continue to be buffered on the PDCP. However, because the PDCP cannot be released, the L2 data transmission function is still unavailable, and the RRC connection reestablishment request fails to be sent.
27. After the RRC T301 expires, all the RBs are released as specified in section 5.3.7.7 and section 5.3.12 of 3GPP TS 36.331.
28. All the buffers of the L2 are released, that is, the memories of the buffers are recovered.
The methods shown in FIG. 1 and FIG. 2 are conventional methods used for recovering memories when the memory of the L2 buffer of the UE overflows. In addition, the two methods depend on the protection of a related protocol timer, and exceptional releasing operations are performed after the timer expires. Because the duration of the related timer and exceptional processing procedures after the related timer expires should comply with the protocol, the whole recovery process may be complex. As a result, the memory recovery takes a long time, and the UE maintains an exceptional state for a long time.